732 research outputs found
The effect of ambipolar electric fields on the electron heating in capacitive RF plasmas
We investigate the electron heating dynamics in electropositive argon and
helium capacitively coupled RF discharges driven at 13.56 MHz by Particle in
Cell simulations and by an analytical model. The model allows to calculate the
electric field outside the electrode sheaths, space and time resolved within
the RF period. Electrons are found to be heated by strong ambipolar electric
fields outside the sheath during the phase of sheath expansion in addition to
classical sheath expansion heating. By tracing individual electrons we also
show that ionization is primarily caused by electrons that collide with the
expanding sheath edge multiple times during one phase of sheath expansion due
to backscattering towards the sheath by collisions. A synergistic combination
of these different heating events during one phase of sheath expansion is
required to accelerate an electron to energies above the threshold for
ionization. The ambipolar electric field outside the sheath is found to be time
modulated due to a time modulation of the electron mean energy caused by the
presence of sheath expansion heating only during one half of the RF period at a
given electrode. This time modulation results in more electron heating than
cooling inside the region of high electric field outside the sheath on time
average. If an electric field reversal is present during sheath collapse, this
time modulation and, thus, the asymmetry between the phases of sheath expansion
and collapse will be enhanced. We propose that the ambipolar electron heating
should be included in models describing electron heating in capacitive RF
plasmas
On the self-excitation mechanisms of Plasma Series Resonance oscillations in single- and multi-frequency capacitive discharges
The self-excitation of plasma series resonance (PSR) oscillations is a
prominent feature in the current of low pressure capacitive radio frequency
(RF) discharges. This resonance leads to high frequency oscillations of the
charge in the sheaths and enhances electron heating. Up to now, the phenomenon
has only been observed in asymmetric discharges. There, the nonlinearity in the
voltage balance, which is necessary for the self-excitation of resonance
oscillations with frequencies above the applied frequencies, is caused
predominantly by the quadratic contribution to the charge-voltage relation of
the plasma sheaths. Using PIC/MCC simulations of single- and multi- frequency
capacitive discharges and an equivalent circuit model, we demonstrate that
other mechanisms such as a cubic contribution to the charge-voltage relation of
the plasma sheaths and the time dependent bulk electron plasma frequency can
cause the self-excitation of PSR oscillations, as well. These mechanisms have
been neglected in previous models, but are important for the theoretical
description of the current in symmetric or weakly asymmetric discharges
Attachment in mental health institutions: A critical review of assumptions, clinical implications, and research strategies
Experimental observation and computational analysis of striations in electronegative capacitively coupled radio-frequency plasmas
Self-organized spatial structures in the light emission from the ion-ion
capacitive RF plasma of a strongly electronegative gas (CF4) are observed
experimentally for the first time. Their formation is analyzed and understood
based on particle-based kinetic simulations. These "striations" are found to be
generated by the resonance between the driving radio-frequency and the
eigenfrequency of the ion-ion plasma (derived from an analytical model) that
establishes a modulation of the electric field, the ion densities, as well as
the energy gain and loss processes of electrons in the plasma. The growth of
the instability is followed by the numerical simulations
Children with disrupted attachment histories: Interventions and psychophysiological indices of effects
Diagnosis and treatment of children affected by disruptions of attachment (out of home placement, multiple changes of primary caregiver) is an area of considerable controversy. The possible contribution of psychobiological theories is discussed in three parts. The first part relates the attachment theoretical perspective to major psychobiological theories on the developmental associations of parent-child relationships and emotional response. The second part reviews studies of autonomic reactivity and HPA-axis activity with foster children, showing that foster children show more reactivity within physiological systems facilitating fight or flight behaviours rather than social engagement, especially foster children with atypical attachment behaviour. The third part is focused on treatment of children suffering from the consequences of disrupted attachment, based on a psychotherapy study with psychophysiological outcome measures. Implications are discussed for theory, diagnosis, and intervention
Young children with significant developmental delay differentiate home observed attachment behaviour towards their parents
Background: The hallmark of attachment is that contact, proximity and relief from stress are sought from specific individuals, laying important groundwork for healthy socioemotional functioning. This study investigated the extent to which differentiated attachment behaviour can be observed in young children with significant developmental delay (DD). Method: Video-taped observations of the parent–child and stranger–child interaction were conducted at home and complemented with questionnaires in 20 families with a child with significant DD (age 2–7 years with an average DD of 49 months). Results: Children displayed more intense and persistent contact-seeking, contact-maintaining and resistant behaviour in the episodes with their parent compared to the episodes with the stranger. Parent-reported secure attachment behaviour was slightly more characteristic towards mother compared to father. Conclusions: Even children with significant DD develop differentiated attachment behaviour. Detailed observations may support parents in identifying the interactions that make the attachment relationship with their child special
Peer victimization in Dutch school classes of four- to five-year-olds: Contributing factors at the school level
This research was designed to examine how factors within young children's environment (e.g., school factors, neighborhood) contribute to explaining peer victimization. The sample comprised 2,003 children (between 4 and 5 years of age) from 98 classrooms in 23 elementary schools in the Netherlands. Teachers were asked to complete a questionnaire on exposure to victimization for each child. Multilevel analyses revealed that gender and social climate of the school were directly related to victimization. Furthermore, results indicated that peer victimization in boys was less prevalent when they attended smaller schools. In low-SES neighborhoods victimization scores were significantly lower when schools had implemented clear antibullying policies. Finally, variation among school classes appeared to be strongly associated with victimization, even more so than variation among schools. These findings support broadening the focus beyond the individual child at risk. © 2009 by The University of Chicago. All rights reserved
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